The present invention relates to rotary fluid pressure devices, and more particularly, to such devices which include an internal gear set, an input-output shaft, and a shaft member for transmitting torque therebetween.
Although it should become apparent from the subsequent description of the present invention that it may be useful with many types and configurations of rotary fluid pressure devices, including both pumps and motors, it is especially advantageous when used in a fluid motor, and will be described in connection therewith.
Also, although the invention may be used with devices having various types of internal gear sets such as those of the crescent type, the invention is especially adapted for use in a device including a gerotor gear set and will be described in connection therewith.
Furthermore, although the invention may be used in devices having various configurations of valving, such as rotating disc valves, it is especially suited for use in devices having hollow spool valves, and will be described in connection therewith.
Fluid motors of the type utilizing a gerotor gear set to convert fluid pressure into a rotary output have become popular and are especially suited for low speed, high torque applications. In most of the commercially available fluid motors of this type, one of the primary factors limiting the torque output capability of the motor is the strength of the drive connection which transmits torque from the orbiting and rotating member (rotor) of the gerotor set to the output shaft of the motor. Typically, this drive connection comprises a set of internal splines defined by the rotor, a set of internal splines defined by an enlarged portion of the output shaft, and a main drive shaft (dogbone) having a set of external splines at each end thereof in engagement with the sets of internal splines. Generally, the internal splines are straight whereas the external splines are crowned to take into account the angle at which the drive shaft is oriented relative to the axis of rotation of the motor. Therefore, although the invention may be used with devices in which the externally-toothed member of the internal gear set merely rotates about its axis, and the dogbone shaft merely rotates about its axis, the invention is especially advantageous when used in a device in which the externally-toothed member both orbits and rotates relative to the internally-toothed member, and the dogbone shaft nutates or wobbles, and the invention will be described in connection therewith.
One of the primary reasons why the spline connections have limited the torque capability of prior art motors is the heat build up which occurs as a result of the engagement between the internal and external splines. The heat build up problem is worsened in fluid motors wherein the rotor of the gerotor set both orbits and rotates and the dogbone must translate this orbital and rotational movement into pure rotational movement of the output shaft. The result is a continual rubbing movement of the external splines against the internal splines, which causes additional frictional heat.
In prior art fluid motors, the internal-external spline connections have not had sufficient lubrication. One reason is that they are frequently located in the end of a blind bore, so that any lubricating fluid which is leaked into the bore is inclined to stagnate, rather than transmit heat and contamination away from the spline connection.
The problem of insufficient lubrication of the spline connection becomes especially serious when the motor is operating at low speeds (for example, in the range of 5-10 rpm), and at high output torque (for example, 2000 in.-lbs.). Under these conditions, the temperature of the spline connection rises, the viscosity of the lubricating fluid drops, and a "break-through" of the oil film may occur, resulting in metal-to-metal contact of the splines. This, in turn, causes even more heat build up, a further decrease in torque capacity and possibly, eventual failure of the spline connection.